A coherent light source, such as a laser, may be used to replace a wideband light (light that encompasses a wide range of wavelengths) source, such as a lamp, and/or a narrowband light (light that encompasses a small range of wavelengths) source, such as a light emitting diode (LED), in projection display systems. When a wideband light source is used in a projection display system, color filters may generally be used to create light with desired colors (wavelengths). The use of a narrowband light source may eliminate the color filter. For example, in a projection display system, such as a microdisplay-based projection display system, narrowband coherent light at desired wavelengths produced by multiple lasers may replace a wideband light produced by an electric arc lamp that requires color filters to produce the desired colors of light. Furthermore, besides a laser's compactness and small form factor, the coherent light from a laser may typically be brighter than light produced by LEDs, may have a small etendue, and may be more efficient in terms of energy consumption. Therefore, a reduction in the size of an illumination system used in the microdisplay-based projection display system may be realized compared with an LED or lamp-based system.
Coherent light may be used to illuminate a digital micromirror device (DMD), a form of microdisplay. The DMD may be a part of a microdisplay-based projection display system. The DMD may contain a large number of micromirrors arranged in an array. The DMD is also commonly referred to as an array of light modulators. The micromirrors in the DMD are typically in one of two states (positions) depending on data from an image being displayed. In a first state, a micromirror may reflect the coherent light onto a display plane, and in a second state, the micromirror may reflect the coherent light away from the display plane. The coherent light reflecting off the large number of micromirrors onto the display plane combines to create the image on the display plane.
When coherent light is scattered by a rough surface, such as a display plane, a modulating spatial noise with high contrast may be produced. The modulating spatial noise, commonly referred to as speckle, may be highly objectionable to viewers. Light fields from each individual scatterer in the display plane may add coherently and sum as phasors, resulting in a randomly varying intensity across the display plane.
Speckle generally originates when a plane phase-front from a coherent light source traverses through a medium with optical path length differences that are less than or equal to a coherence length of the laser, where the coherence length may be expressed as:
      L    =                  c                  Δ          ⁢                                          ⁢          v                    =                                    λ            _                    2                          Δ          ⁢                                          ⁢          λ                      ,where c is the speed of light, λ is the mean source wavelength, and Δλ is the source spectral linewidth. Such path length differences may occur due to surface roughness, screen roughness, scratches, digs, dings, polishing imperfections, and so forth, in optical elements.
Speckle reduction and elimination have received considerable attention from scientists and system designers. Speckle reduction techniques may include light beam steering, which may reduce speckles by rapidly moving the light beam so that the speckles created do not remain stationary. The human eye may then average the speckles, making them less noticeable.
One such technique involves the use of an acousto-optic deflector to steer the coherent light beam using a radio frequency signal. Acousto-optic deflectors offer high speed operation (on the order of a few microseconds) to enable some speckle reduction but they have small rectangular apertures. Galvanometric mirrors may also be used to reduce speckles by steering the phase-front of the coherent light beam. Galvanometric mirrors offer a variety of optical aperture sizes and wide steering angles, but are slower in speed as compared to acousto-optic deflectors and have a non-linear scan response time. Their scan response time is inversely related to the aperture size and hence a larger aperture means a slower response time and vice versa.